A third party to a negotiator who has the authority to dictate an agreement is a/an?a)conciliator b)advisor c)negotiator d)mediator e)arbitrator

It is the Arbitrator (E)

All experts above have already mentioned the right answer.... It's Arbitrator  ... :-)

Its the arbitrator. 

The Answer d) Mediator

While a negotiator represents one party or the other, a mediator is an impartial facilitator with no link to either party in a negotiation. He has no interest in the substance of an agreement itself, but is, rather, concerned with the process of helping the parties reach a satisfactory agreement on their own.

e)arbitrator

E

He/She can act as an Arbitator.